Heretofore, methods for the synthesis of a high silica content zeolite and high silica content zeolites prepared by the methods have been known. For example, JP-B-46-10064 (The term "JP-B" as used herein means an "examined Japanese patent publication") and JP-B-56-49850 disclose a method which comprises the hydrothermal treatment of a reaction mixture containing silica, alumina, an alkaline metal, water and a source of organic nitrogen cation precursor.
Further, methods for the preparation of a high hot water resistance and high silica content zeolite and high silica content zeolites prepared by the methods have been known. For example, JP-A-7-291620 (The term "JP-A" as used herein means an "unexamined published Japanese patent application") discloses a high silica content zeolite having a high percent ion exchange and an excellent resistance to hot water, as compared to conventional high silica content zeolites, which comprises long hexagonal plate crystal particles having a major axis/minor axis ratio of from 2 to 15 and a major axis/thickness ratio of from 4 to 50 in a proportion of not less than 75% by number, and which has a percent crystal retention of not less than 85% when subjected to steaming treatment at 900.degree. C. for 5 hours, obtained by hydrothermally treating an aluminosilicic acid gel having a pH value of from 11 to 13 prepared by simultaneously pouring a silica component and an alumina component into an aqueous solution of a neutral salt in an amount of not less than 7.5 mol per mol of alumina component and a process for the preparation of the high silica zeolite.
Further, JP-A-3-293031 discloses a high heat resistance zeolite having an oxide molar composition of aK.sub.2 O, bNa.sub.2 O, Al.sub.2 O.sub.3, cSiO.sub.2 and dH.sub.2 O which exhibits a crystallinity, as measured after hot water treatment at 900.degree. C. for 5 hours in steam having a water content of not less than 10%, which is not less than 90% in comparison with that as measured before the heat treatment, and also discloses a process for the preparation thereof.
Further, JP-A-3-193622 discloses a process for the preparation of particulate ZSM-5 which comprises crystallizing a mixture of feedstocks containing silica source, alumina source, alkaline metal source and water under hydrothermal synthesis conditions, characterized in that as a seed slurry there is added before crystallization a hydrothermally synthesized half-crystallized slurry in an amount of from 10 to 40% by weight based on the total weight thereof, in which the slurry has a nitrogen adsorption BET surface area of from 100 to 250 m.sup.3 /g, the solid matter contained in the slurry shows an X-ray diffraction pattern of ZSM-5 after dried.
Further, JP-B-7-35343 and JP-B-7-94396 disclose a process for the preparation of an aromatic hydrocarbon from a light hydrocarbon in the presence of an ZSM-5 type zeolite containing zinc as a catalyst, characterized in that the catalyst has a specific silicon/aluminum atomic ratio and zinc/silicon atomic ratio and allows the desorption of pyridine in an amount of from 40 to 120 .mu.mol per g of the ZSM-5 type zeolite at a temperature of from 500.degree. C. to 900.degree. C. by hot desorption method.
However, the high silica content zeolite synthesized by the method described in JP-B-46-10064 and JP-B-56-49850 is disadvantageous in that it has an insufficient crystallinity and hence gives a crystal having a poor heat resistance, causing permanent deterioration of zeolite catalyst by dealuminization in a high temperature atmosphere having water content as that occurred in the combustion and elimination of carbon materials accumulated during the reaction. The high silica content zeolite is also disadvantageous in that a large amount of carbon materials are accumulated thereon during the reaction, so that a large-sized apparatus for combusting and eliminating the carbon materials is necessitated.
The zeolite synthesized by the method disclosed in JP-A-7-291620 and JP-A-3-293031 has a high hot water resistance. However, these patent publications have no reference to the use of the zeolite in a reaction which uses a feedstock containing an aromatic hydrocarbon or which provides a product containing an aromatic hydrocarbon. The problem of activity drop by carbon materials accumulated on the zeolite catalyst during the reaction is not recognized in these publications.
Further, zeolite needs to have a raised crystallinity to raise its hot water resistance. Eventually, such zeolite tends to have a large particle diameter. Such zeolite having a large particle diameter has a small ratio of the number of surface acid sites to the total number of acid sites. Thus, the activity drop due to carbon materials accumulated on the zeolite catalyst during a reaction which uses a feedstock containing an aromatic hydrocarbon or which provides a product containing an aromatic hydrocarbon is accelerated, making it impossible to put such zeolite into practical use.
The zeolite synthesized in JP-A-3-193622 is a particulate zeolite for use in the reaction to produce cyclohexanol from cyclohexene as described in the examples thereof. In this publication, minimization of the particle diameter of zeolite is intended to achieve. Thus, zeolite having a small particle diameter is described in the examples. However, in a reaction which uses a feedstock containing a component which causes an increase of the accumulation of carbon materials, or which reaction gives a product containing such a component, such as the reaction system of the present invention, if the particle diameter of the zeolite is too small, the amount of carbon materials accumulated during the reaction is raised. When the accumulated amount of carbon materials is raised, the time required for the combustion and elimination of carbon materials is prolonged, as compared with the case where the accumulated amount of carbon materials is less, if the same apparatus for the combustion and elimination of carbon materials is used. If the combustion is effected for the same period of time, a larger combustion apparatus is required. In addition, the amount of water content produced per unit time is raised, to thereby accelerate permanent deterioration due to dealuminization. Further, ZSM-5 disclosed in JP-A-3-193622 has a small crystallinity and hence a low hot water resistance that accelerates permanent deterioration of zeolite catalyst due to dealuminization in a high temperature atmosphere having water content.
JP-B-7-35343 and JP-B-7-94396 disclose a process for the preparation of an aromatic hydrocarbon from a light hydrocarbon, characterized in that a catalyst having a specific silicon/aluminum atomic ratio and zinc/silicon atomic ratio and containing a ZSM-5 type zeolite which allows the desorption of pyridine in an amount of from 40 to 120 .mu.mol per g of the ZSM-5 type zeolite at a temperature of from 500.degree. C. to 900.degree. C. by hot desorption method is used, to minimize the activity drop with time due to carbon materials accumulated during the reaction. By predetermining the desorbed amount of pyridine to the above defined range, the activity drop with time due to carbon materials accumulated can be minimized while maintaining the activity required for the production of an aromatic hydrocarbon from a light hydrocarbon. However, these publications give no solutions to the problem of permanent deterioration of the zeolite catalyst due to dealumination in a high temperature atmosphere having water content as that occurred in the combustion and elimination of carbon materials accumulated on the catalyst during the reaction. In any of these publications, as can be seen in the examples, the parameter .alpha. represented by the amount (B) of pyridine desorbed from the catalyst at a temperature of from 500.degree. C. to 900.degree. C. by a hot desorption method when the catalyst is converted into H type after being subjected to a steam treatment at an H.sub.2 O partial pressure of 0.8 atm and a temperature of 650.degree. C. for 5 hours, and the amount (A) of pyridine desorbed from the catalyst at a temperature of from 500.degree. C. to 900.degree. C. by a hot desorption method when converted into H type without the steam treatment, is larger than 1.6. Thus, the deterioration due to deposition of carbon materials for every reaction is reduced. However, these catalysts are liable to rapid permanent deterioration. Thus, these catalysts cannot be practically used for several months or several years in repeating reaction and regeneration by combustion and elimination of carbon materials accumulated during the reaction.
As described above, various methods for the synthesis of zeolite have been proposed. However, no high silica content zeolite-based catalysts have been found excellent in both coking resistance and regeneration deterioration resistance wherein the amount of carbon materials accumulated on the catalyst during a reaction which uses a feedstock containing an aromatic hydrocarbon or which provides a product containing an aromatic hydrocarbon are reduced, temporary activity drop due to carbon materials is inhibited, and permanent activity deterioration of the catalyst due to dealumination in a high temperature atmosphere having water content as that occurred in the combustion and elimination with an oxygen-containing inert gas of the carbon materials is inhibited.